New crust is continually being pushed away from divergent boundaries (where sea-floor spreading occurs), increasing Earth's surface. But the Earth isn't getting any bigger. What happens, then, to keep the Earth the same size? The answer is subduction.
In locations around the world, ocean crust subducts, or slides under, other pieces of Earth's crust. The boundary where the two plates meet is called a convergent boundary. Deep trenches appear at these boundaries, caused by the oceanic plate bending downward into the Earth.
Deep below the Earth's surface, subduction causes partial melting of both the ocean crust and mantle as they slide past one another. This melting generates magma that makes its way to the surface, producing volcanoes, such as Mt. St. Helens. Most of the subducting plate continues into the mantle, perhaps to reappear much later at a distant divergent boundary.
The Sea Floor Spread (divergent boundary)
The Continental Crush (collisional boundary)
Slippin' and a Slidin' (transform boundary)
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